In an emergency a subscriber of a cellular communication system uses his subscriber unit to call an emergency service, such as 911. Often in emergency situations, the subscriber is unable to provide details of his location that would enable an emergency response unit to locate him.
While prior art systems, such as U.S. Pat. No. 5,432,841 by Rimer, discloses a method of determining the location of a subscriber within a cellular system by identifying the location of a base station from which the emergency call was received, the communication coverage area of a base station can be fairly large. In a city, the communication coverage area of a base station can include many buildings.
Consequently, it would be difficult for the emergency response unit to locate the distressed subscriber within the coverage area of the base station in a timely manner.
U.S. Pat. No. 5,613,205 by Dufour describes a method for manipulating the transmit power of base stations neighboring the base station from which the emergency call was received to force the subscriber unit to hand off to the neighboring base stations. Then by determining the power levels at which handoff occurs to each of the neighboring base stations, arcs are drawn around the neighboring base stations. The intersection of the arcs indicates the location of the subscriber unit.
A disadvantage of this method is, power levels of the base stations in a cellular communication system are set by a predetermined algorithm which reduces interference in the system. Any variation to the predetermined algorithm will substantially increase the probability of interference in the system.